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MOTOROLA M68040 USER’S MANUAL 8- 19
any internal registers, but it does cause the M68040 to assert the reset out (RSTO) signal,
resetting all external devices.
8.3 EXCEPTION PRIORITIES
When several exceptions occur simultaneously, they are processed according to a fixed
priority. Table 8-4 lists the exceptions, grouped by characteristics. Each group has a
priority, from 0–7, with 0 as the highest priority.
Table 8-4. Exception Priority Groups
Group/
Priority Exception and Relative Priority Characteristics
0 Reset Aborts all processing (instruction or exception) and does not
save old context.
1 Data Access Error
(ATC Fault or Bus Error)
Aborts current instructions; can have pending trace, floating-
point post-instruction, or unimplemented floating-point
instruction exceptions.
2 Floating-Point Pre-Instruction* Exception processing begins before current floating-point
instruction is executed. Instruction is restarted on return from
exception.
3 BKPT #n, CHK, CHK2, Divide by Zero,
FTRAPcc, RTE, TRAP#n, TRAPV
Illegal Instruction, Unimplemented A- and
F-Line, Privilege Violation
Unimplemented Floating-Point Instruction*
Exception processing is part of instruction execution.
Exception processing begins before instruction is executed.
Exception processing begins after memory operands are
fetched and before instruction is executed.
4 Floating-Point Post-Instruction* Only reported for FMOVE to memory. Exception processing
begins when FMOVE instruction and previous exception
processing have completed.
5 Address Error Reported after all previous instructions and associated
exceptions have completed.
6 Trace Exception processing begins when current instruction or
previous exception processing has completed.
7 Instruction Access Error
(ATC Fault or Bus Error)
Reported after all previous instructions and associated
exceptions have completed.
8 Interrupt Exception processing begins when current instruction or
previous exception processing has completed.
* Refer to Section 9 Floating-Point Unit (MC68040 Only) for details concerning floating-point instructions.
The method used to process exceptions in the M68040 is significantly different from that
used in earlier members of the M68000 processor family due to the restart exception
model. In general, when multiple exceptions are pending, the exception with the highest
priority is processed first, and the remaining exceptions are regenerated when the current
instruction restarts. Note that the reset operation clears all other exceptions except in the
following circumstances:
As soon as the M68040 has completed exception processing for a condition when an
interrupt exception is pending, it begins exception processing for the interrupt
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